Honeycomb structures having a catalyst component loaded thereon have been used in an exhaust gas purifier of a heat engine (e.g. an internal combustion engine) or a burner (e.g. a boiler), a reforming unit of a liquid fuel or a gaseous fuel, and the like. Also, it is known that honeycomb filters are used for capturing and removing the particulate matter contained in a particle-containing fluid such as exhaust gas emitted from a diesel engine.
The honeycomb structures or honeycomb filters used for such purposes have had problems; for example, they are subjected to rapid temperature change by exhaust gas or undergo local heating, an uneven temperature distribution easily appears therein, resultantly they come to have cracks. Particularly when they were used as a honeycomb filter for capturing the particulate matter contained in the exhaust gas emitted from a diesel engine, cracks appeared easily because the carbon fine particles accumulated on the filter must be burnt for removal and it inevitably causes local heating to high temperatures and easily generates a large thermal stress. In this case, the thermal stress is generated because the uneven temperature distribution allows different portions of the honeycomb structure to show different thermal expansion deformations and resultantly the individual portions are restricted by each other and are unable to make free deformation.
It is also known that in producing a large honeycomb structure so as to meet the application purpose, a plurality of honeycomb segments are bonded by a bonding material to obtain a one-piece honeycomb structure or honeycomb filter. In this case as well, the thermal stress generated in the honeycomb structure or honeycomb filter produced must be reduced.
To reduce such a thermal stress, there is disclosed, in, for example, U.S. Pat. No. 4,335,783, a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb segments with a discontinuous bonding material. In this honeycomb structure, however, the thermal stress generated therein could not be reduced sufficiently because no consideration was made to the fact that the generation of thermal stress appears mainly in the vicinity of each periphery of the two end faces where the inlet and outlet of each through-channel exist. Further, since the bonding material is formed discontinuously, the bonding strength between honeycomb segments was not sufficient and the produced honeycomb structure had no sufficient mechanical strength.
In JP-B-61-51240 is proposed a thermal shock resistant rotary heat regenerator obtained by subjecting a ceramic material to extrusion molding to obtain a honeycomb matrix body of honeycomb structure, firing the honeycomb matrix body, then making smooth the outer surface of the fired honeycomb matrix segment, coating, on the smoothened outer surface, a ceramic bonding material having substantially the same mineral composition (after firing) as that of the matrix segment and showing a thermal expansion coefficient different from that of the honeycomb matrix segment by 0.1% or less at 800° C., bonding a plurality of such honeycomb matrix segments to each other into one piece, and firing the one-piece structure. In this thermal shock resistant rotary heat regenerator as well, thermal stress appears mainly in the vicinity of each periphery of the two end faces where the inlet and outlet of each through-channel exist; nevertheless, the honeycomb segments are bonded even at the peripheries of the two end faces; therefore, the thermal stress generated could not be reduced sufficiently.
In the SAE Article 860008 of 1986 is disclosed a ceramic honeycomb filter obtained by bonding a plurality of cordierite honeycomb segments with a cordierite cement. In this honeycomb filter as well, bonding is made even at each periphery of the two end faces where the inlet and outlet of each through-channel exist, similarly to the cases of the above-mentioned honeycomb structure, etc.; therefore, the thermal stress generated could not be sufficiently reduced.
In JP-A-8-28246 is disclosed a ceramic honeycomb filter obtained by bonding a plurality of honeycomb ceramic segments with an elastic sealing material formed by bonding at least a three-dimensionally intertweaved inorganic fiber and inorganic particles via an inorganic binder and an organic binder. In this honeycomb filter as well, the honeycomb segments and the sealing material are not composed of the same material and bonding is made even at each periphery of the two end faces where the inlet and outlet of each through-channel exist; therefore, the thermal stress generated at the end faces could not be reduced.
The present invention has been made in view of such problems of the prior arts. The present invention aims at providing a honeycomb structure which generates no crack caused by the thermal stress during the use and therefore has excellent durability; a honeycomb filter; and processes for producing such a honeycomb structure and such a honeycomb filter.